The present invention relates to a system for production of sterile-packed rice, which is capable of substantially completely eliminating any thermo-resistant bacteria that could remain in rice.
Rice has been known as one of the most healthy foods and taken as the staple food or side-dish ingredient in many countries in the world. Recent consumer lifestyles and dining styles have changed the rice industry environment. In other words, the conventional custom of cooking rice as a domestic work has, at least in part, shifted to the recent trend of buying processed rice products.
Retort-pouch foods have become popular as typical one of processed or pre-cooked foods. A retort-pouch food can be produced by packaging and sealing a processed food in a tray or package and then sterilizing the packed food in a high temperature, high pressure atmosphere. It has been known that when the food temperature rises to above approximately 130.degree. C., any thermo-resistant bacteria that could contain in the food is eliminated substantially perfectly.
However, since heat is transmitted gradually from the peripheral area to the center of the food while the food is subjected to retort sterilization, when the food center reaches a temperature of the order of 130.degree. C., its peripheral area must have much higher temperature, resulting in deterioration of natural food flavor. On the other hand, retort sterilization with a lower temperature prevents flavor deterioration but does not provide a sufficient sterilization because the food center could not reach a predetermined temperature.
Thus, retort-pouch rice can not meet the requirement of perfect sterilization without flavor deterioration. Various attempts have been directed to production of sterile-packed rice that satisfies the above requirements. For example, Japanese patent laid-open publication No. 3-155761 published on Jul. 3, 1991 and Japanese patent laid-open publication No. 3-201954 published on Sep. 3, 1991 disclose sterile-packed rice production system comprising sterilizing a large quantity of washed rice by a high temperature steam, followed by cooking, and packaging and sealing the cooked rice into a plurality of small-size, individual packages. Packaging and sealing operation is carried out in a sterile atmosphere, that is, in a clean room of cleanliness of 100-1000 class. The packages and lids are in advance sterilized by ultraviolet light.
With this system, a huge clean room is needed for packaging and sealing operation. It is laborious and costly to create and maintain a predetermined cleanliness for such a huge clean room. Moreover, workers' hands and clothes must be cleaned or sanitized each time when they go into and out of the clean room, which greatly degrades operation efficiency and increases production cost.